Overweight and obese women — defined as having a BMI (body mass index) higher than 25 — have a higher risk of being diagnosed with breast cancer compared to women who maintain a healthy weight, especially after menopause. Being overweight also can increase the risk of breast cancer coming back (recurrence) in women who’ve been diagnosed with the disease.

This higher risk is partially because fat cells make estrogen; extra fat cells mean more estrogen in the body and estrogen can make hormone-receptor-positive breast cancers develop and grow. Scientists also have recently found that extra fat cells can trigger long-term, low-grade inflammation in the body. Chronic inflammation has been linked to a higher risk of breast cancer recurrence; the proteins secreted by the immune system seem to stimulate breast cancer cells to grow, especially estrogen-receptor-positive breast cancer in postmenopausal women.

Statistics show that more than 66% of U.S. women are overweight or obese, which puts them at higher risk for breast cancer.

A new analysis of information from the Women’s Health Initiative offers some good news: Overweight postmenopausal women who lost about 5% of their body weight — about 10 pounds on average — had a lower risk of breast cancer compared to women who didn’t lose weight.

The research is part of the very large Women’s Health Initiative Observational Study, commonly called the WHI. Overall, the study includes information from more than 161,608 postmenopausal women who were ages 50 to 79 when they joined from 1993 to 1998. None of the women had been diagnosed with breast cancer when they joined the study. The WHI wants to find any links between health, diet, and lifestyle factors and health problems such as cancer.

“We wanted to determine if weight loss was associated with lower breast cancer incidence, as studies have not been able to consistently show that losing weight reduces the risk of breast cancer,” said Rowan Chlebowski, M.D., Ph.D., research professor in the Department of Medical Oncology and Therapeutics Research at City of Hope. Chlebowski is the WHI’s principal investigator.

Earlier studies were limited because they used self-reported weight data. In this analysis, the researchers measured the women’s height and weight and had a longer follow-up time.

This analysis included 61,335 postmenopausal women who had not been diagnosed with any type of cancer. They were age 50 to 79 when they joined the study between 1993 and 1998. At that time, the researchers measured the women’s weight and height and calculated their BMI. Three years later, the researchers remeasured the women’s height and weight and calculated their BMI again.

When the first measurements were taken:

41% of the women were at a healthy weight

34% were overweight

25% were obese

The women were followed for an average of 11.4 years. During that time, 3,061 women were diagnosed with breast cancer.

Compared to women who stayed at the same weight, women who lost weight — a 5% body weight or more weight loss — were 12% less likely to be diagnosed with breast cancer. Women who lost more weight — 15% or more of their body weight — were 37% less likely to develop breast cancer risk.

“With our extensive data collection, we really were able to control for all the variables, including things like metformin use [a diabetes medicine that has been linked to lower breast cancer risk], which was very, very low, and nonsteroidal anti-inflammatory drugs,” said Chlebowski. “So, the characteristics were closely matched. And interestingly, the women who lost weight were heavier and less physically active than the other women; [there were] 8,135 women who lost weight.

“I think it’s a very positive, encouraging message,” he continued. “This weight loss showed that you didn’t have to go down to a normal weight. This was a weight loss that was achievable by women making their own decisions, probably very few of them got into formal programs, and so the idea is that you don’t have to — if you’re trying to say, ‘I will never get back to my normal weight or my weight when I was 22 years old,’ you don’t have to. You just have to lose about 5% and you will have a likely health benefit.”

While gaining 5% or more of body weight wasn’t linked to an overall increase in breast cancer risk, it was associated with an increase of triple-negative breast cancer.

“We have no explanation for why that occurred,” Chlebowski explained. “We’ve thought about that a lot and we don’t have an answer.”

Losing weight can be harder as you get older, but it can be done with careful changes to your diet and daily exercise. The first thing to do is talk to your doctor about a healthy weight for you based on your age, height, body type, and activity level. Next, talk to your doctor about a safe and sensible plan to lose weight designed specifically for you and your needs.

In addition to lower breast cancer risk, losing weight also can help lower any inflammation in your body and also reduce your risk of heart disease and other conditions.